1. Field of the Invention
The present invention relates to a multilayered structure in which insulating material layers and electrode layers are alternately stacked and a method of manufacturing the same. Further, the present invention relates to an ultrasonic transducer including such a multilayered structure and to be used for transmitting and receiving ultrasonic waves in ultrasonic diagnosis and nondestructive inspection.
2. Description of a Related Art
Multilayered structures, in each of which insulating material (dielectric material) layers and electrode layers are alternately formed, are utilized not only for multilayered capacitors but also in various uses such as piezoelectric pumps, piezoelectric actuators, ultrasonic transducers and so on. In recent years, with the developments of MEMS (micro electromechanical systems) related devices, elements each having such a multilayered structure have been microfabricated still further and packaged more densely.
In microfabrication of an element having opposed electrodes, since the smaller the area of the element is made, the smaller the capacity between the electrodes becomes, a problem occurs that the electrical impedance of the element rises. For example, when the electrical impedance rises in a piezoelectric actuator, the impedance matching can not be taken with a signal circuit for driving the piezoelectric actuator and power becomes difficult to be supplied, and thereby, the performance as the piezoelectric actuator is degraded. Alternatively, in an ultrasonic transducer employing a piezoelectric element, detection sensitivity of ultrasonic waves is dropped. Accordingly, in order to enlarge the capacity between electrodes while microfabricating the element, plural piezoelectric material layers and plural electrode layers are alternatively stacked. That is, the capacity between electrodes of the entire element can be made larger by connecting the stacked plural layers in parallel.
FIGS. 23A and 23B are sectional views showing a conventional multilayered structure (piezoelectric device) in which plural piezoelectric material layers and plural electrode layers are stacked. As shown in FIGS. 23A and 23B, in order to connect in parallel the plural electrode layers that sandwich plural piezoelectric material layers 100, interconnection is performed from side surfaces of the multilayered structure.
In the multilayered structure as shown in FIG. 23A, electrodes 101 are formed so that one ends thereof may extend to one wall surface of the multilayered structure, and electrodes 102 are formed so that one ends thereof may extend to the other wall surface of the multilayered structure. Thereby, the electrodes 101 are connected to a side interconnection 103 formed on the one side surface and insulated from a side interconnection 104 formed on the other side surface. Contrary, the electrodes 102 are connected to the side interconnection 104 and insulated from the side interconnection 103. By applying a voltage difference between the side interconnection 103 and the side interconnection 104, an electric field is applied to each of the piezoelectric material layers 100 respectively disposed between the electrodes 101 and the electrodes 102, and the piezoelectric material layers 100 expand and contract by the piezoelectric effect.
By the way, as shown in FIG. 23A, in each layer of the electrodes 101 and 102, insulating regions 105 in which no electrode is formed are provided for insulating the electrodes from either of the side interconnections. The insulating regions 105 do not expand or contract even when a voltage is applied to the multilayered structure 100. On this account, stress is concentrated on this part and this part is easy to break, and therefore, a problem occurs that the reliability of the product becomes low.
In order to prevent such a breakage due to stress concentration, a multilayered structure as shown in FIG. 23B has been proposed. In this multilayered structure, electrodes 111 and 112 are formed over the entire surfaces of the piezoelectric material layers 100. Further, one ends of the electrodes 111 and 112 exposed on the side surfaces of the multilayered structure are covered by insulating materials 115. Thereby, the electrodes 111 are connected to a side interconnection 113 and insulated from a side interconnection 114. Contrary, the electrodes 112 are connected to the side interconnection 114 and insulated from the side interconnection 113.
However, in the multilayered structure as shown in FIG. 23B, since the insulating regions 115 and side interconnections 113 and 114 are formed on the side surfaces, it is difficult to fabricate an arrayed multilayered structure in which a large number of multilayered structures are densely arranged.
By the way, Japanese Patent Application Publication JP-A-6-291380 discloses that a multilayered body is obtained by forming multilayered structure of internal electrode layers and dielectric material layers, and external electrodes by repeating injection deposition of ultrafine particles of internal electrode material, dielectric material and external electrode material by using plural nozzles having different output end forms in a certain order (the fourth page, FIG. 4). By such a fabrication method, a multilayered ceramic dielectric material can be obtained without employing an organic material such as a binder.
The injection deposition method is a film forming method of depositing a raw material by spraying the fine particles of the raw material toward a substrate, and also referred to as “aerosol deposition (AD) method” or “gas deposition method”. In the injection deposition method, the fine particles of the raw material are sprayed at high speed on an under layer such as the substrate or a deposit that has been previously formed, and thereby, a phenomenon called “anchoring” occurs in which the fine particles of the raw material cut into the under layer. At the time of the impingement, a strong film is formed by the mechanochemical reaction in which the fine particles of the raw material are crushed and the crushed faces adhere to the under layer.
In the multilayered structure as shown in FIG. 4 of JP-A-6-291380, not only a lower electrode 2, piezoelectric materials 3 and an upper electrode 4, but also external electrodes 5a and 5b as side interconnections are formed by the injection deposition method. The external electrodes 5a and 5b are required to have thicknesses equal to that of the piezoelectric material in order to connect predetermined interconnections, which are located between the plural piezoelectric materials 3, to each other. However, because nickel (Ni) or palladium silver (Ag—Pd) as a material of the side interconnections 5a and 5b is softer compared to a platinum (Pt) and titanium (Ti) as a material of the lower electrode 2, when the side interconnections are formed by the injection deposition method, anchoring occurs but mechanochemical reaction hardly occurs. On this account, there is a possibility that strong side interconnections cannot be formed. Contrary, it is conceivable that, at this time, ablation (corrosion) occurs and the film deposited once is separated. Further, when the fine particles of the raw material are sprayed from the nozzle, a beam of aerosol (gas in which raw material powder is floating) broadens, and therefore, the edges of the piezoelectric materials become tapered. Accordingly, the repeated formation of multilayers makes the widths of the piezoelectric material layers narrower, and thereby, it is difficult to fabricate an ideal column piezoelectric material. Furthermore, in the case where electrodes are located on the side surfaces of the piezoelectric materials, it becomes difficult to package a large number of microstructures with high density.